1. Field of the Invention
The present invention pertains to sensors such as for toxic compounds in gases or fluids and, in particular, to the manufacture and use of a detector for sensing ammonia.
2. Description of the Related Art
Toxic gas sensors are relied upon to an ever increasing extent to safeguard the health and safety of personnel entering a possibly hazardous area. With added attention being directed to worker's safety in confined spaces, there is a need for rapidly identifying gaseous or liquid agents, which (even if not asphyxiating or combustible) are nonetheless toxic, using compact, portable equipment. With an increasing integration of electronics, the ability to obtain an instrument read-out of toxic sensors is becoming simpler. However, there remains a need to provide sensors having improved accuracy and sensitivity to avoid inappropriate conclusions drawn from a sensor's output indication.
Electrochemical methods are frequently used for analyzing traces of gases. In methods of this kind, the measuring electrode of a galvanic cell is brought into contact with the gas to be examined, producing an electrical current which is proportional to the concentration of the compound to be measured. In the case where reducible gases (for example, oxygen, nitrogen dioxide, ozone or chlorine) is measured, there is a reaction on a measuring electrode which acts as a cathode, while oxidizable gases (for example, hydrogen sulfide, carbon monoxide, hydrogen) react on a measuring electrode acting as an anode.
Many electrochemical ammonia sensor cells in use today employ the Sevringhaus (potentiometric) Principle (DS-A-2009937). A pH glass electrode is employed as a measuring electrode,and the potentials between the pH electrode and a reference electrode are measured. The difference in potential serves as a measurement signal, related to the presence of ammonia through a pH-shift of the electrolyte according to the reaction NH.sub.3 +H.sub.2 O.fwdarw.NH.sub.4.sup.+ +OH.sup.-. This process suffers drawbacks in that the signal is logarithmically related to the ammonia concentration. Moreover, the time to fix the balance is extremely slow. Further, other gases (e.g., SO.sub.2, HCl, CO.sub.2) are able to change the pH-value of the electrolyte, falsifying the measurements.
Other commercially available ammonia sensors (such as those available from Sensoric GmBH & Co., operate on the amperometric principle. Sensors of this type respond to a direct transformation of ammonia passing through a gas-permeable membrane onto a catalytic working measurement electrode, according to the following equation: EQU NH.sub.3.fwdarw.N.sub.2 +6H.sup.+ +6e
Electrodes in these types of sensors include measure, reference and counter-electrodes in contact with a water-free organic gel electrolyte, cooperating with the electro catalyst to oxidize the ammonia to nitrogen. It is important for direct transformation that the electrolyte be a water-free medium. One example of a highly effective catalyst is platinum black. The reaction causes an electric current that is proportional to the concentration of ammonia in the measuring gas. Unfortunately, the oxidation rate of ammonia to nitrogen is not fast enough at higher concentrations, and intermediate by-products of the oxidation of ammonia causes the measuring electrodes to become partly blocked, resulting in a temporary poisoning of this electrode, with a continuous decline in the measuring signal if the ammonia gas is not withdrawn from the sensor. Furthermore, the selectivity of this sensor is not very high.
Another type of amperometric measurement procedure is known from GB 2,225,859. The ammonia passes a gas-permeable membrane into an electrolyte containing soluble non-oxidizable reagent. The reagent changes through a reaction with ammonia to a substance that is electrochemically oxidizable, preferably an organic ammonium salt such as hydrochloride of Tris(hydroxymethyl)-aminomethane ("Tris-HCl"). This transforms the ammonium salt into Tris(hydroxymethyl)-aminomethane, which is oxidized in a second stage, in place of the ammonia itself on the measuring electrode. A highly effective catalyst, either rhodium or gold, must be used with this procedure to produce oxidation of the newly formed amine. Unfortunately, other substances, such as by-products in the measuring gas (e.g., CO or alcohol) will also be measured. Another disadvantage is that a bias-potential is necessary to cause a reaction. This causes long warm-up periods, dependable on temperature, degrading the measuring behavior of the sensors, and impairing the ability to obtain a favorable zero-noise level. In this type of procedure, ammonia reacts only with a particular substance (Tris-HCl) added to the electrolyte, producing an electrochemical active species which later transforms itself at a measuring electrode, creating a measurement signal proportional to ammonia concentration.
A similar measuring procedure is known (U.S. Pat. No. 5,234,567) which uses a chemical species which reacts with ammonia to form a product which is more electrochemically active than ammonia. The chemical species is one of iodine or Nesslers reagent or a solution of manganese and silver nitrate. The preferred chemical reagent is iodine. The ammonia diffuses into the sensor and dissolves readily in water to product OH.sup.- (reaction 1), which is necessary for a secondary reaction forming iodine-ions (reaction 2). EQU NH.sub.3 +H.sub.2 O.fwdarw.NH.sub.4.sup.+ +OH.sup.- (1) EQU 6OH.sup.- +3I.sub.2.fwdarw.5I+IO.sub.3 +3H.sub.2 O (2) EQU 2I.sup.-.fwdarw.I.sub.2 +2e (3)
The measuring signal results from the reformation of iodine from the iodine-ion (reaction 3). This must occur at an elevated potential of the measuring electrode. A +300 mV bias potential is required between the reference and measuring electrodes. This requires a long warm up time to stabilize the zero reading of the sensor. Another disadvantage is that a very small side reaction can also form iodine-ions from iodine (reaction 4). EQU I.sub.2 +3H.sub.2 O.fwdarw.5I.sup.- +IO.sub.3.sup.- +6H.sup.+ (4)
This side reaction is temperature dependent with a small increase of temperature substantially increasing the zero current, which again is a big disadvantage.